In the art of detergency, granular laundry detergents containing admixed sodium carbonate have been found to exhibit poor water solubility under certain conditions. For example, in countries which typically launder clothes in relatively cold temperature (e.g. 2.degree. C. to 30.degree. C.) washing solutions, the solubility of carbonate-containing granular detergent compositions has not been satisfactorily complete. After undergoing a conventional laundering process in these situations, solid masses or "clumps" of detergent ranging from about 5 to 40 mm in diameter and about 2 to 10 mm in length remain in the washing machine and deposited on the laundered clothes. Such clumps usually occur when the detergent is placed in a pile, particularly during cold temperature washes and/or when the order of addition to the washing machine is laundry detergent first, clothes second, and water last (the so-called "reverse order of addition"). This solubility problem is easily identifiable by consumers of such granular detergents and is commercially unacceptable.
It has been known that the primary contributor to the aforementioned solubility problem is the admixed sodium carbonate in the granular laundry detergent. While not intending to be limited by theory, it is believed that this solubility problem is caused by hydration of the sodium carbonate, which results in a sticky, poorly soluble solid mass, before the granular detergent can be completely dispersed and dissolved in the laundering solution. It is important to note that this problem is normally only associated with granular detergents containing "admixed" sodium carbonate, i.e. carbonate which is subsequently mixed with or added to the base granule. By contrast, granular detergents containing sodium carbonate which forms part of the base granule by way of being added to the crutcher and spray dried in conjunction with the other base granule ingredients typically does not experience the cold water solubility problem to which the present invention is directed.
As is known, citric acid has been used as a builder in granular laundry detergents. Additionally, citric acid has been used in some cases to increase the water solubility of granular detergents containing sodium carbonate and the like by way of a chemical reaction between the citric acid and sodium carbonate to release carbon dioxide. However, the citric acid ingredient has long been regarded as a relatively expensive component of modern day granular detergents and thus only low levels have been used to produce detergent products economically acceptable for consumers. Additionally, citric acid is known to be hygroscopic and thus contribute to undesirable caking of the detergent product prior to use by the consumer. It would therefore desirable to have a lower cost material which would provide the solubility benefits desired and reduce or eliminate the need for the citric acid component in granular detergents.
Accordingly, it would be desirable to have a granular detergent composition which exhibits improved water solubility, especially at cold temperatures. It would also be desirable to have such a granular detergent which is less expensive.